Limit switches or phototubes having a light source and a light detector which receives the light from the light source are generally used for detecting the passage of a stock to be cut in a carpenter planer. The use of such a type of phototubes, however, involves the limitation of the width of the stock to be cut although the stock is fed on a feeding table having an open edge. As a result, reflection type optical detectors have been recently adopted which are not subjected to this limitation. The reflection type optical detector, however, has another disadvantage that the passage of a stock to be cut may not be accurately detected when the stock has a mortise or rough surface.